Hitherto, it has been known that capsaicin of the following formula(A) ##STR2## which may be extracted from the fruit of a plant belonging to the genus Cayenne has an analgesic property. Such natural capsaicin(i.e., trans-8-methyl-N-vanillyl-6-nonenamide) and a synthetic capsaicin (N-vanillyl-nonanamide) have been described in U.S. Pat. No. 4,313,958 as an analgesic. Also, similar results of chemical and pharmacological studies have been reported in various references [see, e.g., Yaksh, et al., Science, 206, pp 481-483(1979); and The Alkaloids, Vol. XXIII, pp 227-299(1984), Academic Press].
Described in EP Publication No. 0,282,127 as having anti-inflammatory or analgesic activity are the aminoethoxybenzylamine derivatives of the following formula(B) ##STR3##
Further, U.S. Pat. No. 4,424,205(EP Publication No. 0,089,710) filed by T. R. LaHann et al. discloses the analgesic and/or anti-irritant compounds of the following formula(C) ##STR4## wherein R.sup.a is a linear or branched C.sub.3 -C.sub.11 alkyl, alkynyl or aralkyl group, a linear or branched C.sub.3 -C.sub.22 alkenyl group, or an unbranched or branched C.sub.6 -C.sub.11 cycloalkyl or cycloalkenyl group; and one of R.sub.1 and R.sub.2 is OH, the other being OH or H.
U. K. Patent Publication No. 2,168,975 filed by J. M. Janusz also provides aralkanamides having formula(D) ##STR5## wherein R.sup.b represents H, OH, OCH.sub.3 or OCH.sub.2 CH.sub.3 ; R.sup.c represents H or CH.sub.3 ; and n is an integer from 0 to 12.
Gardner et al. generically discloses in U.S. Pat. No. 5,045,565 .beta.-aminoethyl-substituted phenyl compounds, including a compound of formula(E): ##STR6## wherein, R.sup.d is a hydrogen, or a hydroxy or methoxy group; and
R.sup.e is a C.sub.1-24 alkyl group. PA1 Y, which may be the same or different when p is greater than 1, is a hydrogen, halogen, methylenedioxy, hydroxy, trifluoromethyl, R.sup.3 or OR.sup.3 wherein R.sup.3 is an optionally substituted C.sub.1-8 alkyl or benzyl group; PA1 n is an integer from 1 to 6; and PA1 p is an integer from 1 to 5.
According to the detailed description of the Gardner patent the term "alkyl" is defined to include straight carbon chains optionally substituted with aryl, which is, in turn, defined to include a phenyl optionally substituted with a halogen, hydroxy, C.sub.1 -C.sub.16 alkoxy, amino, nitro, cyano, phenyl, benzyl, benzyloxy, trifluoromethyl, formylamino, carboxylate or C.sub.1 -C.sub.6 alkyl group.
Although the Gardner patent discloses alkyl substituted phenyl acetamide derivatives of the above formula(E), it fails to, e.g., describe N-n-alkyl substituted phenyl acetamides with a phenyl group attached to the end of the n-alkyl group.
The foregoing prior art capsaicin derivatives can be characterized by their having a saturated or unsaturated alkyl group of 3 to 24 carbon atoms or an unsubstituted .omega.-phenyl alkyl group attached to the carbonyl group. These compounds are, however, generally known to have several side-effects: for example, strong irritation, reddening of skin and toxicity.